Identification and qualification of temperature signal ...

ARTICLE IN PRESS

Optics and Lasers in Engineering 44 (2006) 1348–1359

Identification and qualification of temperature signal for monitoring and control in laser cladding Guijun Bia,, Andres Gasserb, Konrad Wissenbachb, Alexander Drenkerb, Reinhart Popraweb a Innovative Manufacturing Processes Group, School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, UK b Fraunhofer Institute for Laser Technology-ILT, Aachen, Germany

Received 26 October 2005; received in revised form 20 January 2006; accepted 23 January 2006 Available online 10 March 2006

Abstract Laser cladding has been successfully introduced into industry for the use in wear and corrosion applications and in the repair work such as turbine components, moulds and dies. Through monitoring and furthermore controling the cladding process, the quality and reproducibility in the production can be ensured. Thus the economic efficiency can increase through the reduced scrap rate. The aim of this work is to identify and analyse the infrared temperature signal emitted from the melt pool, which could be used for quality control and for closed loop control. Different measure systems including a photodiode, pyrometer and CCD camera with different functional wavelengths were used to detect the temperature radiation. The detected signals show dependence on the main process parameters including laser power, powder feeding rate and scanning speed. The results of the clad such as dilution and dimension have very good correlation with the measured temperature signal. A process monitoring and control system based on the infrared temperature signal with coaxial alignment of the ancillary lenses was established and tested successfully. r 2006 Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Infrared temperature signal; Process monitoring and control

Corresponding author.

E-mail address: [email protected] (G. Bi). 0143-8166/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2006.01.009

ARTICLE IN PRESS G. Bi et al. / Optics and Lasers in Engineering 44 (2006) 1348–1359

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1. Introduction Laser cladding with powder shows several advantages [1] over other traditional surface cladding technologies such as PTA and TIG. These advantages enable the wide industrial applications of this technology for improving corrosion, erosion and wear resistance as well as repair and modification of expensive machine parts. Cladding of serial products like outlets and valve seats in the automotive industry has been also successfully realized [2]. One of the most essential requirements to adopt this technique in industrial manufacturing is to fulfil the increasing demands on the product quality. Thereby through the decreased scrap rate the economic efficiency can be significantly increased. This needs to develop a method to monitor and furthermore control the cladding process online. For laser cladding, laser power PL, powder feeding rate mp and scanning speed vv are the most important process parameters which govern the quality and geometry of the clad. On the one hand, the temperature signal depends on the different process parameters, measuring devices and alignments of the ancillary lenses as well as geometry and material of the cladded component; on the other hand, the temperature in the melt pool also influences the geometry and properties of the clad. Some research has been done to detect and monitor the temperature signal with contact thermal couple [3], monochromatic [4–6] and quotient pyrometers [7–9] as well as CCD cameras [10–12] in laser material processing. But there are no systematic studies on the effect of the main process parameters on the temperature signal in laser cladding. No comparative research with different measuring systems and different alignment of the ancillary lenses has been carried out. In this paper, the temperature signal in the infrared (IR) range was identified and qualified through experiments. The influence of the main process parameters on the signal was studied. Different measuring systems including photodiode and quotient pyrometer were used as a comparison to detect the temperature signal. The influence of different alignments of the ancillary lenses on the measurement was investigated. A CCD camera was also applied to capture the size of the melt pool. Based on the fundamental study, a process monitoring and control system was established and tested.

2. Temperature measurement with a radiant thermometer Due to the high temperature in the melt pool for laser cladding (normally 1200–2600 1C [13]), it is ideal to measure the temperature signal with a non-contact method. This can be achieved with radiant thermometers. Advantageously, they are suitable for measuring the temperature under some extreme conditions, especially for industry applications [14]. Solids, fluids, gases and plasmas send electromagnetic radiations, which are called thermal radiation or temperature radiation. The radiation depends on the temperature of the radiator. Fig. 1 shows schematically the temperature measurement by laser material processing with a radiant thermometer. Based on Planck’s

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G. Bi et al. / Optics and Lasers in Engineering 44 (2006) 1348–1359

Fig. 1. Emission and measurement of the temperature radiation by material processing with laser.

law, the spectral radiance Ll;S ðl; TÞ of a black radiator can be defined by the following equation: c c1 2 Ll;S ðl; TÞ ¼ 5 exp , (1) lT l where Ll;S , spectral radiance of the black radiator (mW sr1 mm3); l, wavelength (mm); c1, first radiance constant (119.1 mW mm2 sr1); c2, second radiance constant (14388 mm K); T, temperature (K). The spectral radiant flux F from the interaction zone to the detector depends on several factors. It arises from the integration of temperature distribution Tð~ xÞ, the emission ratio e, the spectral radiance Ll;S of the black radiator, the cosine of the emission angle W and the solid angle O over the measurement spot AM as presented in Eq. 2: Z Fl ðl; AM ; . . .Þ ¼ eðl; Tð~ xÞÞLl;S ðl; Tð~ xÞÞ cos Wð~ xÞO dAM , (2) AM

where Fl, spectral radiant flux (W); Tð~ xÞ, temperature distribution; e, emission ratio; r, direction of the emission; W, emission angle; O, solid angle; AM: measure spot. Because the signal is conveyed to the detector through a set of optical system, the losts of the signal must be taken into account. Thus the detector signal can be defined by integration of the spectral radiant flux and the spectral sensitivity over the whole spectral range (Eq. 3). Z 1 SðAM ; . . .Þ ¼ Rl ðlÞ Fðl; AM ; . . .Þ dl, (3) 0

where S, detector signal; Rl, spectral sensibility of the optical system. For the purpose of integrating the sensor system in the optical path of the laser beam, the sensor must be sensitive in the transmission range of the glass optics (400–2600 nm). Systems based on photodiode, pyrometer and CCD camera are


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frequently applied. The main difference between the simple photodiode and pyrometer exists in the calibration. Pyrometers will be calibrated with a reference radiator to absolute temperature and display the measured temperature in Celsius.

3. Experimental procedures 3.1. Experimental set-up Fig. 2 shows the whole experimental set-up. This system consists of a 4-axis NC-machine, an HL 3006 cw Nd:YAG laser with highest output power of 3 kW and a YC 50 cladding head from the company Precitec. As displayed, the laser beam was guided through an optical fibre from the laser machine to the workplace and focused by an optic with 200 mm focal length. The diameter of the circular laser spot adopted in the experiments was 2 mm at the surface of the workpiece. A homogeneous powder jet was guaranteed by the coaxial powder nozzle. The powder used was ironbase martensitic stainless-steel METCO 42C in the size of 45–105 mm in diameter. Table 1 shows the composition of the powder. The material of the substrate was mild carbon steel C45. A germanium photodiode with a functional wavelength in the range of 1200–1600 mm was integrated in the cladding head. The ancillary lenses of this sensor were aligned coaxially to the laser beam. A quotient pyrometer TCS whose

Fig. 2. Experimental set-up.


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Table 1 Composition of METCO 42C powder Element

C

Cr

Fe

Mn

Ni

P

S

Si

Weight %

0.20

15.9

80.67

0.1

2.43

0.01

0.01

0.7

Fig. 3. Schema of the cross-section.

ancillary lenses were laterally aligned with an angle b ¼ 451 to the laser beam, was also used simultaneously as a comparison to detect the temperature signal in the IR range. One Si and one InGaAs detector were applied in TCS. Their functional wavelengths were 900 and 1550 nm, respectively. Additionally, the melt pool was observed by a CCD camera which was coaxially aligned to the laser beam. The camera has the highest sensibility at 650 nm and an image rate of 33 frames per second. 3.2. Arrangement of the experiments The influence of the main process parameters on the IR-temperature signal was studied by cladding single tracks in a length of 40 mm. All measured signals were arithmetically averaged. The standard deviation was calculated. The average and the standard deviation were used for presenting the results. After the experiments, the samples were cut vertical to the scanning direction and in the middle of the track, then polished, etched and photographed under microscopy. Fig. 3 shows schematically the evaluation of the cross-section of the cladded track. The dilution Z was calculated with equation 4: Z¼

HD . HC þ HD

(4)

The correlation between the measured signals and the clad quality such as defects, dilution and dimension were studied in detail.


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4. Results and discussion 4.1. Influence of laser power on the IR-temperature signal Laser power was first varied intentionally in order to study its influence on the IR-temperature signal. Meanwhile, the powder feeding rate and the scanning speed were kept constant at 3.2 g/min and 500 mm/min, respectively. Fig. 4 shows the IR-temperature signals measured by the quotient pyrometer TCS and the Ge photodiode as a function of the laser power. The photos of the melt pool and the cross-section of the cladded tracks are shown in Fig. 5. Both curves of the temperature signal show a similar trend: between 300 and 800 W the signal increases more rapidly than between 800 and 1100 W. A slight increase in the measured temperature signal with increasing laser power has been reported in Ref. [7] by laser alloying and dispersion. Fig. 4 also displays the dimension of the cladded track in cross-section and the size of the HAZ plotted against laser power. The width of the track and the HAZ, and the diameter of the melt pool display a strong dependence on the laser power. The

Fig. 4. Influence of the laser power on the measured IR-temperature signal, dimension of the cladded track, HAZ and size of the melt pool.

Fig. 5. Photos of the melt pool and cross-sections of the cladded tracks by varying laser power.



Fig. 6. IR-temperature signal as a function of dilution.

curves bear resemblance to the temperature signal. However, the laser power has a very slight influence on the height of the cladded track. Furthermore, the diameter of the melt pool correlates with the width of the track except at 500 W. Because the laser beam owns a Gaussian form of power intensity, at 500 W the power intensity at the border of the laser spot did not suffice to clad the powder onto the substrate surface. And the powder in this area only glowed. However, the whole highlighted area triggered the CCD camera and was recorded. Thus the track width deviates significantly from the diameter of the recorded melt pool. The highlighted area of the melt pool was acquired by the CCD camera and characterized as the diameter of melt pool (vertical to the scanning direction). The calibration of the CCD camera was not carried out. At 300 W power no clad was generated, because the laser power was not sufficient to melt the fed powder. As a result, there was no signal detected by the CCD camera. From 900 W power the dilution increases obviously. In Fig. 6, the IR-temperature signal is plotted as a function against dilution. The signal increases only slightly if dilution is more than 7%. When the dilution is high, the process changes to laser alloying. A thorough blend of the melted powders with the melted base material occurred as shown in the cross-section. Thus a large part of the heat is conducted away rapidly through the substrate, which results in the insignificant increase in the measured IR-temperature signal. 4.2. Influence of powder feeding rate on the IR-temperature signal To study the influence of powder feeding rate on the IR-temperature signal, this parameter was varied step by step. The laser power and the scanning speed were maintained constantly at 800 W and 500 mm/min, respectively. Fig. 7 shows the influence of the powder feeding rate on the measured IR-temperature signals, dimension of the cladded track and size of the melt pool. The signal detected by TCS rises slightly till 3.2 g/min powder feeding rate, then keeps nearly constant till 8.6 g/ min and decreases slightly afterwards. This is concordant with Refs. [4–6] by laser cladding. In comparison, the signal measured by the Ge photodiode increases continually till 8.6 g/min powder feeding rate and then keeps nearly constant for the


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Fig. 7. Influence of powder feeding rate on the measured IR-temperature signal, dimension of the cladded track and size of the melt pool.

Fig. 8. Photos of the melt pool and cross-sections of the cladded tracks by varying the powder feeding rate.

higher powder feeding rate. The result of the Ge photodiode is concordant with that in Ref. [7] by alloying and dispersing but contradicts that in Refs. [4–6] by laser cladding. The height of the track changes considerably by varying the powder feeding rate which can be also recognized in Fig. 8. It increases rapidly with increasing powder feeding rate till 8.6 g/min, then alters only slightly with a higher powder feeding rate. Excluding the first measurement, the width of the track decreases slightly with increasing powder feeding rate till 8.6 g/min, and then remains nearly constant. Note that the circular laser beam has a Gaussian distribution of the power intensity; the power intensity at the border of the laser spot did not suffice to melt and clad all the fed powders completely. This can be proved by the pores and boning defects in the clad at a high powder feeding rate. The diameter of the melt pool shows no dependence on the powder feeding rate, but correlates with the diameter of the characterized laser spot. The difference between the two curves of the Ge photodiode and TCS results from the lateral alignment of the ancillary lenses of TCS. The same lateral alignment was also applied in Refs. [4–6], as shown schematically in Fig. 9. By layer 1 (low powder feeding rate), the measure spot of TCS is adjusted at position 1, in the middle of the melt pool. As the thickness of the cladded layer increases by increasing the powder



Fig. 9. Schema of the measurement at different layer thickness.

feeding rate, the measure spot shifts to position 2. That means the measure spot deviates from the middle of the melt pool, whereby a lower temperature signal will be measured than the real temperature. This proves that the lateral alignment of the ancillary lenses can result in significant measure error. To be concerned, for threedimensional cladding, the geometry change of the melt pool which results from the change of the scanning direction will influence the measurement significantly with this kind of set-up. The measurement will be strongly direction dependent. Therefore lateral alignment of the ancillary lenses is not suitable for laser cladding. 4.3. Influence of scanning speed on the process signals Scanning sped was varied from 250 to 1000 mm/min with an interval of 250 mm/ min in order to study its influence on the IR-temperature signal. The laser power and the ratio of powder feeding rate and scanning speed mp/vv were kept constant at PL ¼ 800 W and mp/vv ¼ 6.4 g/m. A constant ratio of mp/vv may guarantee nearly the same height of every single track. Fig. 10 shows the influence of the scanning speed on the measured IR-temperature signal, dimension of the cladded track and size of the melt pool. It shows that the IR-temperature signal decreases slightly with increasing scanning speed. The same conclusion has been reported in the literature [7] for laser alloying and dispersion. Fig. 11 displays the belonging cross-sections of the cladded tracks and photos of the melt pool. Very high dilution occurred solely at vv ¼ 250 mm/min. It resulted from the increasing ratio of laser power and scanning speed PL/vv, while the scanning speed decreases. It can also be identified that the width of the track and diameter of the melt pool decrease with increasing scanning speed, but the height remains almost constant. This can also be explained by the decreasing ratio of laser power and scanning speed PL/vv. In general, the measured IR-temperature signal has dependence on the main process parameters. The laser power shows the strongest influence on the signal. The lateral alignment of the ancillary lenses can result in obvious measure errors. The results proved that

the IR-temperature signal is suitable for process monitoring and control, and coaxial alignment of the ancillary lenses is ideal for temperature measurement in laser cladding.


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Fig. 10. Influence of scanning speed on the measured IR-temperature signal, dimension of the cladded track and size of the melt pool.

Fig. 11. Photos of the melt pool and cross-sections of the cladded tracks by varying the scanning speed.

5. Applications A process monitoring and control system was built based on the IR-temperature signal. The photodiode which was integrated in the cladding head with coaxially aligned ancillary lenses was used. The system was tested by cladding thin axes in a diameter of 7 mm. Firstly the experiment was carried out with a constant laser power. Then the IR-temperature control with different set values was trialed. Fig. 12a shows the results of cladding with a constant laser power in the general view, longitudinal section and the belonging signals of laser power and IR-temperature. The axis display an oxidation-free and an oxidized area, respectively. From the longitudinal section, it can be observed that the dilution and the thickness of the clad increase with increasing oxidation. In the area where the



Fig. 12. Surface view, longitudinal section of the cladded axis and signals of the laser power and the IRtemperature (a) without process control and (b) with process control.

surface is oxidation free and the dilution is very low, the IR-temperature signal increases significantly. The gain ends as soon as the heavy oxidation and dilution appear. The quality of the clad can be recognized from the measured IR-temperature signal. Furthermore, a control system based on the IR-temperature signal was set up and tried. As discussed above, the laser power shows the strongest influence on the IR-temperature signal, thus laser power was adopted as a variable. Based on the measured temperature signal, the sound set value of the temperature signal should lie between 0.3 and 0.6 V, because in this range the cladded axis shows low dilution and no oxidation. Fig. 12b displays the results by using 0.4 V set value. It can be identified that the temperature signal first increases until it reaches the set value and then keeps constant. The laser power stays at 0.7 V first and then decreases continuously. The surface of the clad and the longitudinal section show that there is no oxidation and defects in the cladded layer. Meanwhile the dilution is very low. Such kind of thin axis can only be cladded without defects using a closed-loop control system. 6. Conclusions Through the experiments, the influence of the main process parameters on the IR-temperature signal have been studied and discussed in detail. Different measure systems and methods have been applied and compared. An IR-temperature monitoring and control system has been established and tested. In general, the following conclusions can be drawn: (1) The detected IR-temperature signal has clear dependence on the main process parameters. The signals measured by TCS and the Ge photodiode exhibit similar dependence on laser power and scanning speed. (2) The lateral alignment of the ancillary lenses can result in significant measure error and are not suitable for laser cladding. Thus coaxial alignment is strongly recommended. (3) The laser power shows the strongest influence on the IR-temperature signal. When low dilution is achieved, the signal increases significantly with increasing laser power.


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(4) A process monitoring and control system based on IR-temperature signal was built and tested successfully. It verified the possibility for further industrial applications.

References [1] Gasser A, Stromeyer R. Hingezaubert: Laser-Auftragschweissen im Wergzeugbau, 10. Wuerzburger Wergzeugtage, Laser und mehr, 27–28 September 2000, Wuerzburg, Germany [in German] [2] Kawasaki M, Takasse K, Nakagawa M, Mori K, Nemeto M, Takagi S, et al. The laser cladding application for automotive parts in Toyota. In: Proceedings of the 25th Internation Symposium on automotive technology and automation (ISATA), 1992, Florenz, Italy [3] Hu YP, Chen CW. Measurement of temperature distribution during laser cladding process. J Laser Appl 2000;12(3):126–30. [4] Backes G, Kreutz EW, Gasser A, Stromeyer R, Wissenbach K. Process monitoring and control during alloying and cladding with CO2 laser radiation. European conference on laser treatment of materials (ECLAT), 22–23 September 1998, Hannover, Germany, p. 227–36. [5] Backes G, Gasser A, Kreutz EW, Trojan K, Wissenbach K. Processdiagnose beim Beschichten mit CO2-Laserstrahlung. Internationale Kongresses Laser 91 [in German]. [6] Backes G. Prozeskontrolle und -regelung beim Legieren. Beschichten und Dispergieren EU 194 Abschlussbericht, Fkz 13 EU 00061/1-8,1995 [in German]. [7] Klocke F, Scheller D. Process monitoring in laser surface treatment operations with reflection and temperature measurement. Prod Eng 1997;4(1):39–44. [8] Ignatiev M, Smurov I, Flamant G. Real time optical pyrometry in laser machining. Meas Sci Technol 1994;5:563–73. [9] Smurov I. Pyrometry applications in laser machining. Proc SPIE 2001;4157:55–66. [10] Hu D, Wu Y, Kovacevic R. Heat input control in 3 D laser cladding based on infrared sensing. In: Proceedings of 2001 ASME international mechanical engineering congress and exposition, MED-vol. 12. 11–16 November 2001, New York NY. P. 333–41 [11] Legrand AC, Meriaudeau F, Gorria P. Bispectral temperature measurements in the steel industry. In: Proceedings of the international symposium on advanced sensors for metals processing. 22–26 August 1999, Que´bec, Canada, p. 317–29. [12] Doubenskaia M, Bertrand Ph, Smurov I. Optical monitoring of Nd:YAG laser cladding. Thin Solid Films 2004;453–454:477–85. [13] Olliver B, PIR-ch N, Cladding with laser radiation—properties and analysis laser treatment of materials. DGM Informationsgesellschaft Verlag, 1992. [14] Drenker A. Adaptive Messung und Regelung der Temperatur beim Umwandlungsha¨rten mit CO2Laserstrahlung. PhD thesis, Aachen University of Technology, Germany, Verlag der Augustinus Buchhandlung, 1997, p. 10 [in German].